In recent years and continuing, distributed reflector (DR) lasers that enable single longitudinal mode oscillation have been used in high-speed fiber-optic transmission systems. As a distributed reflector laser, a semiconductor laser that integrates a distributed feedback (DFB) laser diode configured to oscillate upon current injection and a distributed Bragg reflector (DBR) laser diode serving as a reflector without current injection, is being developed. In the DFB region, only a specific wavelength is selected from the light produced at the active layer by the diffraction grating structure and this specific wavelength light reciprocates in the DFB region. The DBR region is used as a reflecting mirror provided at the rear end of the DFB active region. The diffraction grating of the DBR region reflects the specific wavelength of light.
As illustrated in FIG. 1, a distributed reflector laser is generally designed such that the lasing mode of the DFB laser comes at the center of the reflection spectrum of the DBR mirror. However, when the index of refraction increases due to the thermal effect by current injection, the Bragg wavelength of the DFB laser shifts to the long-wavelength side (“red shift”) and mode hopping may occur. Moreover, when the index of refraction decreases due to carrier absorption (plasma effect), the Bragg wavelength of the DFB laser shifts to the short-wavelength side (“blue shift”) and multi-mode oscillation may occur.
Some structures for preventing mode hopping or multi-mode oscillation are known. For example, the grating period of the diffraction grating of the DBR mirror is varied in the direction of the resonator to increase the DBR reflection bandwidth. See, for example, Japanese Laid-open Patent Publication No. 2010-251609. Another known technique is to bend the lines (or grooves) of the diffraction grating of the distributed reflecting mirror region such that the end parts of the lines extending in the width direction of the diffraction grating are more away from the active region than the center of the grating. In this structure, the coupling coefficient of the diffraction grating is kept constant in the width direction of the diffraction grating. See, for example, Japanese Laid-open Patent Publication No. 2011-135008.